The present invention relates to a silica-group composite oxide fiber having excellent photocatalyst function, electrical function and/or thermal-catalyst function and having high strength and a process for the production thereof. Specifically, the present invention relates to a fiber which is composed of a central portion (silica phase) covering mechanical properties and an oxide phase constituting a surface layer and a layer close to the surface layer which oxide phase covers various functions and which has a slope constitution toward the surface layer and a process for the production thereof.
Many studies have been carried out concerning an photocatalyst effect on a semiconductor, typified by titanium dioxide, since the latter half of the 1970s stating with decomposition reaction of water. When the above photocatalyst effect is utilized, conventionally, titania crystal grains are fixed on a substrate and used. However, many problems occur concerning a bonding method so that in recent years a keen attention is focused on a titania fiber free from the fixation problem.
For example, JP-A-5-184923 discloses a method of synthesizing a fiber composed of crystals of anatase type titania and vanadium oxide which method comprises dissolving titanium alkoxide and a vanadium compound in alcohol, carrying out hydrolysis to prepare a sol-like material, forming the sol-like material into a fiber-like material, gelating the fiber-like material and heat-treating the gel in the range of from 200 to 700xc2x0 C. In examples of the above JP-A-5-184923, there is mainly described a fiber containing titania and vanadia and further containing quantities of a silica component. With regard to catalyst activity as a fabric using the above fiber, there is shown only catalyst activity of a fabric obtained by mixing only 20% of the above fiber into an E glass fiber made of silica.
Conventionally, it is known that a titania fiber synthesized by the sol-gel method is extremely fragile. As a study for increasing the strength thereof, for example, xe2x80x9cYogyo-Kyokai-shixe2x80x9d, vol 94 (12), pages 1,243 to 1,245, (1986) describes the coexistence of a silica component. The above method described in examples of JP-A-5-184923 exactly adopts this method. Further, JP-A-11-5036 publication discloses a silica-titania fiber for a photocatalyst according to the sol-gel method and a production process thereof. In this case, likewise, the fiber has an extremely low strength of 0.1 to 1.0 GPa.
In addition to the above methods, there are the following reports as a production process of titania. For example, xe2x80x9cJournal of Material Science Lettersxe2x80x9d 5 (1986), 402-404, reports a method of synthesizing a gel-like titania fiber (anatase) in which hydrochloric acid coexists in an alcohol solution of titanium alkoxide, hydrolysis is carried out to obtain a colloidal substance and the colloidal substance is spun, heated under a humidified atmosphere and then temperature-increased in air to obtain the gel-like titania fiber.
Further, xe2x80x9cThe American Ceramic Society Bulletinxe2x80x9d, May 1998, 61-65, reports a method of producing a titania fiber by adding water to fine particles of titania to obtain a slurry, mixing the slurry with viscose to prepare a viscous fluid, forming the viscous fluid into a fiber and calcining the fiber in air under heat at a high temperature.
Each of these fibers is formed through an agglomeration step of primary particles of titania so that the inside of each fiber has a serious defect. Even when a photocatalyst function is recognized, therefore, it is extremely fragile. Accordingly, it is required to solve many problems for practical uses. Further, in systems where a silica component coexists in order to improve strength, titania and silica exist in a mixed state so that these systems can not provide sufficient photocatalyst activity when compared with titania alone. This is also a significant problem for practical uses.
When a photocatalyst fiber is used as a filter, it is naturally preferred that the photocatalyst fiber has a higher fiber strength since the photocatalyst fiber is exposed to a high-speed gas flow for a long period of time. Particularly, in consideration of its application to a gas emitted from an aircraft engine or a motor vehicle engine, it is strongly desired to develop a fiber having high-strength photocatalyst function or thermal-catalyst function that goes beyond conventional common sense.
It is an object of the present invention to provide a composite oxide fiber having a metal oxide phase (first phase) covering mechanical properties and having a metal oxide phase (second phase) expressing various functions and mainly composed of a metal different from a metal oxide of the first phase on its surface layer and in a region close to the surface layer and a process for the production thereof.
It is another object of the present invention to provide a composite oxide fiber having excellent strength and having excellent effect of expressing various functions such as photocatalyst function or thermal-catalyst function and a process for the production thereof.
According to the present invention, there is provided a silica-group composite oxide fiber formed of a composite oxide phase of an oxide phase (first phase) mainly made of a silica component and a metal oxide phase (second phase) excluding silica, in which the existent ratio of at least one metal element of a metal oxide constituting the second phase upward slopingly increases toward the surface layer of the fiber.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the existent ratio of the first phase based on the fiber as a whole is 98 to 40% by weight and the existent ratio of the second phase based on the fiber as a whole is 2 to 60% by weight.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the oxide phase mainly made of a silica component contains a metal element or a metal compound which can form a solid solution or a eutectic compound with the silica.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the metal oxide phase excluding silica is made of titania, its eutectic compound or an lead/zirconium/titanium type oxide.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the slope in the existent ratio of at least one metal element of a metal oxide constituting the metal oxide phase excluding silica exists from the fiber surface to a depth in the range of 5 nm to 500 nm.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the slope in the existent ratio of at least one metal element of a metal oxide constituting the metal oxide phase excluding silica exists to a depth of ⅓ of the diameter of the fiber.
According to the present invention, further, there is provided a silica-group composite oxide fiber according to above, wherein the metal oxide of the second phase is titania which has a crystal particle diameter of 15 nm or less and has photocatalyst and/or thermal catalyst functions.
According to the present invention, further, there is provided a process for the production of the silica-group composite oxide fiber recited above, which process comprises melt-spinning a modified polycarbosilane having a structure obtained by modification of a polycarbosilane having a main chain structure represented by the formula, 
in which R is a hydrogen atom, a lower alkyl group or a phenyl group and n is an integer of 1 to 30,
and having a number average molecular weight of 200 to 10,000 with an organic metal compound, or a mixture of the modified polycarbosilane and an organic metal compound to obtain a spun fiber, infusibilizing the spun fiber, and then calcining the infusible fiber in air or in oxygen.
According to the present invention, further, there is provided a process recited above, wherein the organic metal compound is a compound having a basic structure of the formula M(ORxe2x80x2)n or the formula MRxe2x80x3m,
in which M is a metal element, Rxe2x80x2 is an alkyl group having 1 to 20 carbon atoms or a phenyl group, Rxe2x80x3 is acetyl acetonate, and each of m and n is an integer of more than 1.
According to the present invention, further, there is provided a process according to above, wherein the modified polycarbosilane is a product in which the organic metal compound is bonded to the polycarbosilane as a monofunctional polymer.
According to the present invention, further, there is provided a process according to above, wherein the mixture is formed of the modified polycarbosilane and a low-molecular weight organic metal compound which is a monomer, a dimer or a trimer.
According to the present invention, further, there is provided a process according to above, wherein the modified polycarbosilane contains a modified polycarbosilane having a low molecular weight.